Using ffmpeg to split mkv but get a few seconds video lost (cutting between keyframes without re-encoding?)

command lineffmpegmatroska

I am using ffmpeg (the real one from FFmpeg, not the fake one from Libav) to split some mkv files:

ffmpeg -i input.mkv -ss 00:01:00 -to 00:02:00 -codec copy output.mkv

However, when I open the output.mkv with VLC, the first few seconds has no video, but only audio. And seconds later I can see both video and audio. The interesting part is, when I try different timestamps after -ss parameter, the duration of "audio only" part varies from 1 second to 4 seconds or even more.

I try to use mkvmerge with --split "timecodes:00:01:00,00:02:00", and the output file works fine, with no "audio only" part at the beginning.

I tried different mkv container with different video/audio encoding in them, but the results are same.

I think I am missing some options with ffmpeg. What should I add to ffmpeg command-line to avoid the several seconds of "audio only" part?

Update 1

Here is the complete output:

$ ffmpeg -i 21.mkv -ss 00:00:58 -to 00:02:00 -codec copy clip.mkv
ffmpeg version 2.2.2 Copyright (c) 2000-2014 the FFmpeg developers
  built on May  7 2014 13:08:45 with gcc 4.8.2 (GCC) 20140206 (prerelease)
  configuration: --prefix=/usr --disable-debug --disable-static --enable-avresample --enable-dxva2 --enable-fontconfig --enable-gnutls --enable-gpl --enable-libass --enable-libbluray --enable-libfreetype --enable-libgsm --enable-libmodplug --enable-libmp3lame --enable-libopencore_amrnb --enable-libopencore_amrwb --enable-libopenjpeg --enable-libopus --enable-libpulse --enable-librtmp --enable-libschroedinger --enable-libspeex --enable-libtheora --enable-libv4l2 --enable-libvorbis --enable-libvpx --enable-libx264 --enable-libx265 --enable-libxvid --enable-pic --enable-postproc --enable-runtime-cpudetect --enable-shared --enable-swresample --enable-vdpau --enable-version3 --enable-x11grab
  libavutil      52. 66.100 / 52. 66.100
  libavcodec     55. 52.102 / 55. 52.102
  libavformat    55. 33.100 / 55. 33.100
  libavdevice    55. 10.100 / 55. 10.100
  libavfilter     4.  2.100 /  4.  2.100
  libavresample   1.  2.  0 /  1.  2.  0
  libswscale      2.  5.102 /  2.  5.102
  libswresample   0. 18.100 /  0. 18.100
  libpostproc    52.  3.100 / 52.  3.100
Input #0, matroska,webm, from '21.mkv':
    encoder         : libebml v1.3.0 + libmatroska v1.4.0
    creation_time   : 2014-05-07 01:47:41
  Duration: 00:42:50.78, start: 0.000000, bitrate: 2833 kb/s
    Stream #0:0(eng): Video: h264 (High), yuv420p(tv, bt709), 1280x720, SAR 1:1 DAR 16:9, 23.98 fps, 23.98 tbr, 1k tbn, 47.95 tbc (default)
    Stream #0:1: Audio: ac3, 48000 Hz, 5.1(side), fltp, 384 kb/s (default)
Output #0, matroska, to 'clip.mkv':
    encoder         : Lavf55.33.100
    Stream #0:0(eng): Video: h264 (H264 / 0x34363248), yuv420p, 1280x720 [SAR 1:1 DAR 16:9], q=2-31, 23.98 fps, 1k tbn, 1k tbc (default)
    Stream #0:1: Audio: ac3 ([0] [0][0] / 0x2000), 48000 Hz, 5.1(side), 384 kb/s (default)
Stream mapping:
  Stream #0:0 -> #0:0 (copy)
  Stream #0:1 -> #0:1 (copy)
Press [q] to stop, [?] for help
frame= 1414 fps=0.0 q=-1.0 Lsize=   18275kB time=00:01:02.00 bitrate=2414.6kbits/s    
video:15344kB audio:2906kB subtitle:0 data:0 global headers:0kB muxing overhead 0.134916%

Update 2

There may be something to do with "keyframe" stuff, and letting ffmpeg re-encode the video stream will not lead to the few seconds of blank video. However, since mkvmerge can cut the same file between keyframes without re-encoding, I am wondering if there is a way to make ffmpeg do the same work. For example, somewhat "re-define" the keyframe, et cetra.

Best Answer

In order to cut in between keyframes (or GOP's, as they are called), you need to re-encode the video. (the same is true for audio, but there the "frames" are much smaller, usually 1024 samples).

In your case, you could change -codec copy to -acodec copy -vcodec webm to have ffmpeg re-encode your stream. This will obviously take longer, and you will lose quality (if you do it right, this loss will not be noticeable, but there will be some).